When a mouse tumor is grown in a host that can mount an immune response against it the dose of X-rays required for cure may be reduced by as much as one half. The interaction of X-rays and immunity in the killing of tumor cells is being studied in the C3H mouse ascites sarcoma BP8. Normal C3H mice, nominally syngeneic with the tumor, can be immunized against lethal inocula of viable BP8 cells. This immunity can be measured either by survival after viable cell challenge or by use of a method in which tumor cells are prelabeled with the thymidine analog 125-I-labeled iododeoxyuridine and followed after injection into mice undergoing immune responses. Loss of radioactivity is monitored by whole body gamma counting and this loss can be correlated with loss of tumor cells. These methods are being used to study the evolution of actively-induced tumor immunity and its effects on tumor cells sublethally damaged by X-irradiation.